1. Field of Invention
This invention relates to windmills as structures harnessing wind to generate power.
2. Description of Prior Art
Windmills have been in existence for thousands of years. However, all the designs have one thing in common-they only have one axel. Blades are fixed to the axel and the axel revolvers inside a hub. A tower usually elevates the hub while allowing a large tail behind the hub to rotate the windmill axel directly into the wind. Wind passing around a plurality of blades creates drag or lift that forces the axel to rotate inside the hub. This design is simple to construct but it has several limitations.
First, single-axis windmills are not as mechanically efficient as possible. At high tip velocity, blades cutting perpendicular into the wind, generating more than 10 times more power than drag devices. Therefore, the most efficient windmill would have blades designed for pure lift all along the blade. In contrast, single-axis windmills provide high velocity only at the tip of the blade. The remaining blade has linearly decreasing velocity as it approaches near zero at the hub. To generate power at lower velocity, the angle of cut into the wind is varied along the blades (twist) producing considerable drag rather than the much more efficient lift force.
Secondly, single-axel windmills have poor economy of scale. It is desirable to increase power per windmill with proportionally less cost and less land. A logical economy of scale is to increase wind intersection by adding more blades. However, with single-axel windmills adding more blades can actually lower power generation by stalling the wind (excessive solidity). To avoid this problem, power windmills typically limit the number of blades to three or less and increase wind intersection by making the blades longer. Longer blades not only provide more wind intersection but also produce higher velocity for a given wind speed. But the highest blade velocity is still only at the tip, and the rest of the blade still suffers from decreasing velocity along the blade length. Also, long blades sweep so much area that they actually have too little solidity, grossly under utilizing available wind and land. Furthermore, as the windmills become larger, more weight is added to the top of the tower, requiring it to be stronger and more costly to build.
Third, starter motors are impractical because it would need to be collocated with the generator, adding too much weight and bulk to the top of the tower. Thus, windmill efficiency is often compromised for higher-drag design that starts easier.
Forth, the blades are complex, expensive, and dangerous. The blades must be very strong to absorb all the centrifugal force that dramatically increases with blade-length and tip velocity. Furthermore, the varying angle of attack along the blade length (twist) is complex and expensive.
Fifth and final, existing windmills can damage the environment. While windmills are a clean source of energy, the long, fast moving blades are often invisible to birds, killing thousands each year.
In accordance with the present invention, multiple axels provide higher power-generating efficiency by enabling blade configurations where maximum blade velocity is achieved across the full length of the blade not just at the tip. The less-efficient middle of single-axel windmills is eliminated.
This windmill is economically scalable to very large size. The base supports all the windmill components, eliminating the need for large towers supporting massive weights at their tops. Regardless of windmill size, the mechanical stress on the blades is low because blades are mechanically supported at both ends. The design allows as many blades and at whatever size that optimize solidity, land-use, and power output.
Starter motors are practical since it can easily be located on the base, allowing a windmill design for maximum power.
Blades are simple. Designed for pure lift, there is no blade twist. Supported at both ends, the blades can be lighter without breaking apart.
The rim has a dual function as the magnet for the generator, reducing parts and increasing power output. The rim can be magnatized and sectioned to have different alternating magnetic poles that pass through wire coils fixed to the base, generating electrical power in the coils.
Finally, this windmill protects birds because the many blades surrounded by supports are easily seen and avoided by birds.
Accordingly, advantages a multi-axel windmill has over a single axel windmill are:
More power-efficiency because full blade length rotates at maximum velocity.
Scalable to large power because more weight and more blades are easily supported.
Starter motor permits use of optimum-efficient, pure-lift blades.
No blade twist and double-end support enables simpler, lighter, and cheaper blades.
More power output from fewer components because the rim is also the generator magnet.
Multiple blades surrounded by supports are easily seen and avoided by birds.